[dpdk-dev] [RFC] More changes for rte_mempool.h:__mempool_get_bulk()

[Ananyev, Konstantin]

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Wiles, Roger Keith
> Sent: Sunday, September 28, 2014 6:52 PM
> To: <dev at dpdk.org>
> Subject: [dpdk-dev] [RFC] More changes for rte_mempool.h:__mempool_get_bulk()
> 
> Here is a Request for Comment on __mempool_get_bulk() routine. I believe I am seeing a few more issues in this routine, please look
> at the code below and see if these seem to fix some concerns in how the ring is handled.
> 
> The first issue I believe is cache->len is increased by ret and not req as we do not know if ret == req. This also means the cache->len
> may still not satisfy the request from the cache.
> 
> The second issue is if you believe the above code then we have to account for that issue in the stats.
> 
> Let me know what you think?
> ++Keith
> ---
> 
> diff --git a/lib/librte_mempool/rte_mempool.h b/lib/librte_mempool/rte_mempool.h
> index 199a493..b1b1f7a 100644
> --- a/lib/librte_mempool/rte_mempool.h
> +++ b/lib/librte_mempool/rte_mempool.h
> @@ -945,9 +945,7 @@ __mempool_get_bulk(struct rte_mempool *mp, void **obj_table,
>                    unsigned n, int is_mc)
>  {
>         int ret;
> -#ifdef RTE_LIBRTE_MEMPOOL_DEBUG
> -       unsigned n_orig = n;
> -#endif

Yep, as I said in my previous mail n_orig could be removed in total.
Though from other side - it is harmless.

> +
>  #if RTE_MEMPOOL_CACHE_MAX_SIZE > 0
>         struct rte_mempool_cache *cache;
>         uint32_t index, len;
> @@ -979,7 +977,21 @@ __mempool_get_bulk(struct rte_mempool *mp, void **obj_table,
>                         goto ring_dequeue;
>                 }
> 
> -               cache->len += req;
> +               cache->len += ret;      // Need to adjust len by ret not req, as (ret != req)
> +

rte_ring_mc_dequeue_bulk(.., req) at line 971, would either get all req objects from the ring and return 0 (success),
or wouldn't get any entry from the ring and return negative value (failure).
So  this change is erroneous.

> +               if ( cache->len < n ) {

If n > cache_size, then we will go straight to  'ring_dequeue' see line 959.
So no need for that check here.

> +                       /*
> +                        * Number (ret + cache->len) may not be >= n. As
> +                        * the 'ret' value maybe zero or less then 'req'.
> +                        *
> +                        * Note:
> +                        * An issue of order from the cache and common pool could
> +                        * be an issue if (cache->len != 0 and less then n), but the
> +                        * normal case it should be OK. If the user needs to preserve
> +                        * the order of packets then he must set cache_size == 0.
> +                        */
> +                       goto ring_dequeue;
> +               }
>         }
> 
>         /* Now fill in the response ... */
> @@ -1002,9 +1014,12 @@ ring_dequeue:
>                 ret = rte_ring_sc_dequeue_bulk(mp->ring, obj_table, n);
> 
>         if (ret < 0)
> -               __MEMPOOL_STAT_ADD(mp, get_fail, n_orig);
> -       else
> +               __MEMPOOL_STAT_ADD(mp, get_fail, n);
> +       else {
>                 __MEMPOOL_STAT_ADD(mp, get_success, ret);
> +               // Catch the case when ret != n, adding zero should not be a problem.
> +               __MEMPOOL_STAT_ADD(mp, get_fail, n - ret);

As I said above, ret == 0 on success, so need for that change.
Just n (or n_orig) is ok here.

> +       }
> 
>         return ret;
>  }
> 
> Keith Wiles, Principal Technologist with CTO office, Wind River mobile 972-213-5533

NACK in summary.
Konstantin